It occurred to me that it would take quite a few posts and time to remember and document every little tweak and hack I've used on my phone. Since part of the fun is in experimentation and reading up on what others have done I direct you to a wonderful site:
Do you want to find some neat open source applications? Go there.
Do you want to find some nifty hacks? Go there.
Do you need some help with troubleshooting a problem? Go there.
Do you want to install a completely different ROM or operating system? Go there!
I've never tried to upgrade my ROM, yet, but I've heard great things. That and so much more can be found at XDA. It's an awesome resource. Check it out.